somethign that has always bothered me is that when scientists talk about exploring the galaxy, because we can only _see_ what is out there but can't actually _be_ out there we have to put up with only knowing whats going on in the past (after a certain distance). Since it is an understatement to state that scientists are interested in the past of the universe they wouldnt be too upset about this. But I really would like to know whats going on _now_ as opposed to what happened millions (or thousands) of years ago.

Id also like to know how far we can see before we start getting into the distant past (more than 10 years).

Thanx

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From: Martin Smith

4/02/99 12:10:24

Subject: re: state of the universe

post id: 273

Everything we see happened in the past. The amount of time that has passed since it happened to when we see it is the time the light took to reach us.

Light travels at around 3*10^8 m/s.

When you look at the sun you are seeing it as it was about 8 minutes ago.

A light year is the distance light travels in a year. So if a star is 10 lights years away the light reaching us left that star 10 years ago.

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From: Tony

4/02/99 12:15:03

Subject: re: state of the universe

post id: 277

yeah I know all that, I wasnt asking WHY it is, I was asking why no one seems to care.Im just getting a bit tired of hearing how things were. I wanna know how things are.

And ten lights years away doesnt mean much im afraid. Gonna nead a reference of some sort.

Thanx

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From: Jason from Hobart

4/02/99 12:25:41

Subject: re: state of the universe

post id: 283

The answer to the last part of your (original) question: 10 light years in distance from the observer is stuff that happened 10 years ago.

Jason

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From: Dr. Ed G (Avatar)

4/02/99 12:49:57

Subject: re: state of the universe

post id: 294

I think you're confusing what the media tells you about what scientists are doing with what scientists are actually doing. The sort of stuff that gets reported is just a small subset of the total work that's actually being done.

I'm not complaining about the media necessarily (although lack of scientific nous on the part of the media is a factor that should be addressed), it is also many scientists' fault for not jumping up and down (jumping through hoop and stuff) in order to get their research "out there".

However, you should understand that generally only the stuff that can be associated with good "vision" (that's media lingo for exciting or pretty pictures, video, etc.), and virtual images of an artists rendition of the Universe always pulls the viewer. Indeed I once saw an article about the CSIRO which was based exclusively around the fact that they were tearing down a glass greenhouse to build a new building - it had great "vision" of bulldozers smashing through the glasshouse, but virtually no important science.

Scientists are doing much more to understand what's going on here and now (particularly with the many evironmental challenges that face humanity), we simply need to learn to communicate with the public that funds us better! (which is the reason I take time to answer questions here)

Soupie twist,Ed G

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From: Chris (Avatar)

4/02/99 13:16:04

Subject: re: state of the universe

post id: 296

10 light years is the distance light travels through vacuum in 10 years.

10LY = (10 x 365.25 x 24 x 3600)s x 300 000 000m/s

= 9.467 x 10^16m = 94 670 000 000 000km.

Why doesn't it matter that we can only see into the past? Because special relativity forbids anything outside our past light cone from interacting with us. Our "past light cone" is simply the range of events in space and time which can reach us with a light ray. In space, this is basically "anything we can see". At present there may well be an enormous turtle devouring stars in a galaxy 10 billion light years away, but no consequence of that astral lunch can affect us for 10 billion years... cause it would take at least that long to get here.

Hence our lack of interest. Events which have occured in the past 10 billion years at a distance of less than 10 billion years may effect us now. Hence our interest.

Hope this helps!Chris

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From: tony

4/02/99 14:18:01

Subject: re: state of the universe

post id: 320

it helps in one way in that it gives me a perspective into what soem people are thinking. however....

firstly that lot of numbers (which Im quite capable of working out myself btw) is about as meaningful as just saying 10 light years. There just numbers dude and way too big to be meaningful to anyone but an astro physicist. They just mean "BIIIIIG NUMBER". What I was more looking for was what kinds of bodies are within that range and have w taken a close look at them (i assume we have if theyare there).secondly, explorations sole purpose is not to idnetify dangers but to see whats there just because it _is_ there. So what if its not gonna affect us. We should want to know whats there and what its doing now anyway. Just because they are there. I'd guess this is affected by funding.

As for the light cone bit. Sure we can see it from here. Have we launched anything out there tha can send stuff back even if it does take decades to get back?

Thanxfor the reply,tonyTony

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From: James Richmond

4/02/99 17:31:56

Subject: re: state of the universe

post id: 365

Basically, we're all stuck in our own "now". If I stand 1 metre away from you, there is no way you can known anything about what is happening to me "now" before 1 nanosecond or so after it happens (which is the time light takes to travel from me to you). Nor can anything you do affect me for at least 1 nanosecond after you do it. In a sense, you can are constantly interacting with the world as it was, not as it is, since you only see things that have already happened.

It gets worse as we go to greater distances. If the sun exploded now, there is absolutely no way we could know about it or be affected by it for 8.5 minutes. When we look further out into space, its not that we wouldn't like to know what's happening now. Rather, we have no option but to observe the past and wait until the light has had time to reach us.

Worse still, "now" is a relative concept. If you stay still and I walk around the room, my perception of which events are simultaneous becomes slightly different to yours, due to relativistic effects. At higher speeds, this becomes more noticable, and can even lead to disagreements on which of two events happened first.

But is this really worth worrying about? After all, there's nothing we can do to change things. Might as well just put up with it.

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From: Chris (Avatar)

4/02/99 18:07:57

Subject: re: state of the universe

post id: 374

Tony wrote:firstly that lot of numbers (which Im quite capable of working out myself btw) is about as meaningful as just saying 10 light years. There just numbers dude and way too big to be meaningful to anyone but an astro physicist. They just mean "BIIIIIG NUMBER". What I was more looking for was what kinds of bodies are within that range and have w taken a close look at them (i assume we have if theyare there).

Hmmm… I was sure you were not interested in light years – but I’ll let that pass. Ten light years is sufficient distance to include our entire solar system, and our nearest stellar neighbours: (distances in light years, you may work out the km if you wish…)

secondly, explorations sole purpose is not to idnetify dangers but to see whats there just because it _is_ there. So what if its not gonna affect us. We should want to know whats there and what its doing now anyway. Just because they are there. I'd guess this is affected by funding.

No it is not funding, and you misunderstand me if you think I meant “danger” when I said “effect”. This is not a matter of choice, it is a fundamental fact of relative space-time. The simple common understanding of “simultaneity” which we encounter everyday is not a useful tool for understanding large distances and times. When I say “effect” I mean it is not possible for any signal to travel to us from an object separated in space but not in time. The term for such an event is that it causally separated from us – it cannot interact with us, nor we with it. As such, speculation about it is as sensible as wondering if there is a great chicken “outside” our universe – ie you may wonder all you like, you can’t test it without waiting for the event to enter your past light cone.

It is meaningless in a relativistic universe to consider what a far distant object might be like “now” when “now” is a local concept. Time is not absolute – that notion died out early this century. (local time, of course.. ;o)

As for the light cone bit. Sure we can see it from here. Have we launched anything out there that can send stuff back even if it does take decades to get back?

Hmmm… you’ve missed the point again. It is not a matter of technology, with the Hubble telescope we can see a very long way. However you necessarily look back in time as you look away in distance. You will have to do away with the naïve notion that space and time are separate – you can not see something in space until the light from it has time to reach you. End of story.

Hope this helps! Chris

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From: Brendan

4/02/99 19:55:45

Subject: re: state of the universe

post id: 392

Chris,

I was just reading your answer to this question and noted your detailing Barnard's Star as being 6 light years from earth.

I was therefore wondering if you knew whether or not Barnard's Star is the nearest "Sun-like" star to the Sun? I had heard/read (can't remember which!) that it was.

Best wishes,BrendanPS: Who was Barnard?

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From: Chris (Avatar)

5/02/99 11:26:26

Subject: re: state of the universe

post id: 444

Actually, no it isn't. Barnard's star is spectral class M5 - it is a main sequence dwarf star, smaller and less bright than the sun.

Alpha A Centauri is closer than Barnard's star, has the same spectral class as our sun (G2), comparable mass (1.1 solar mass) and radius (1.23 solar radii). That would make it very similar to the sun... except that it is part of a binary system (with alpha B centauri) which orbits with a third star (proxima centauri).

Hope this helps!Chris

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From: Craigus

5/02/99 11:53:30

Subject: re: state of the universe

post id: 447

Chris,I was under the impression that Proxima centauri was the closest star to earth (our sun, yes I know). If Alpha A&B centauri are orbiting Proxima centauri, wouldn't they at some stage be closer to us, in our unfashionable end of the milky way, absolutely nothing like the Neptune/Pluto turnout?(You know, the same but different)Burger ring casseroleCraigus

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From: tony

5/02/99 13:07:00

Subject: re: state of the universe

post id: 450

Hmmm… I was sure you were not interested in light years – but I’ll let that pass. Ten light years is sufficient distance to include our entire solar system, and our nearest stellar neighbours: (distances in light years, you may work out the km if you wish…)